In many cases, steel plates having a thickness of 8 mm or more are rapidly cooled by quenching or accelerated cooling after hot rolling and then subjected to tempering treatment in order to achieve high strength and toughness.
In recent years, while the quenching or the accelerated cooling is carried out in an on-line manner, the tempering treatment is carried out in a gas combustion furnace as usual in an off-line manner, so that it takes time to considerably decrease the productivity of steel plate. Therefore, several methods have been proposed to increase the productivity in the tempering treatment process.
To increase the productivity, for example, JP-A-9-256053 proposes a new heating pattern of tempering treatment. In this heating pattern, the steel plate is continuously transferred in a gas combustion furnace, which is set to high temperature at the inlet side thereof, and to low temperature at the outlet side thereof, to be subjected to heat treatment. To be concrete, the temperature at the inlet side of the furnace is set to be higher by at least 200° C. than the target heat treatment temperature so that the furnace temperature is decreased stepwise toward the outlet side of the furnace, and the temperature at the outlet side of the furnace is set to be within ±20° C. from the target heat treatment temperature. In a heating system by gas combustion, however, the heat transfer is conducted through radiation and convection, so that the rapid heating is impossible and the productivity can not be adequately increased.
JP-A-4-358022 and JP-A-6-254615 propose, as a heat treatment method for increasing the productivity, an on-line heat treatment method, in which a heating apparatus is installed on a rolling line to heat the steel plate. The former discloses a method for manufacturing a steel plate having high strength and toughness, in which a rolling mill, an accelerated cooling apparatus, and a heating apparatus are arranged on a rolling line to perform rapid heating in tempering treatment, and the latter discloses a method, in which a rolling mill, a leveler, an accelerated cooling apparatus, and a″heat keeping furnace are arranged on a rolling line so that the residual stress in steel plate, generated by rolling and accelerated cooling, is removed in the heat keeping furnace. However, these methods cause problems that it takes time for heat treatment and that the steel plate can not be uniformly subjected to heat treatment.
JP-A-48-25239 discloses a method for heat treating a steel plate, in which a plurality of solenoid type induction heating apparatuses are arranged in series on a rolling line. When the steel plate is heated to the target temperature with a single induction heating apparatus, the induced current is localized in the vicinity of the steel plate surface, thus sometimes causing a problem that the surface thereof is excessively heated to over the Curie point or the Ac1 transformation point. Therefore, since it is difficult to heat the thickness-wise center of steel plate to the target temperature with a single induction heating apparatus during controlling the surface temperature of steel plate to or below a certain temperature, JP-A-48-25239 arranges two or more induction heating apparatuses in series, and passes a steel plate through these apparatuses so that the surface temperature of the steel sheet does not exceed the upper limit temperature and the thickness-wise center temperature reaches the target temperature. For that purpose, however, it is necessary to decrease the transfer speed of steel plate, so that it is not possible to attain the adequately high productivity. In particular, in case of the steel plate having a large thickness, it takes time for heating the thickness-wise center of the steel plate, and therefore the transfer speed has to be decreased to a large extent, so that the productivity is remarkably decreased.